AVIATION MAPS - JMAPS - AVIATION MAPS

NDB navigation consists of two parts — With the automatic direction finder (or ADF) equipment on the aircraft that detects an NDB's signal, and the NDB transmitter. The ADF can also locate transmitters in the standard AM medium wave broadcast band.

When tracking to or from an NDB, it is also usual that the aircraft track on a specific bearing. To do this it is necessary to correlate the RBI reading with the compass heading. Having determined the drift, the aircraft must be flown so that the compass heading is the required bearing adjusted for drift at the same time as the RBI reading is 0 or 180 adjusted for drift. An NDB may also be used to locate a position along the aircraft track. When the needle reaches an RBI reading corresponding to the required bearing then the aircraft is at the position. However, using a separate RBI and compass, this requires considerable mental calculation to determine the appropriate relative bearing.

To simplify this task a compass card is added to the RBI to form a "Radio Magnetic Indicator" (RMI). The ADF needle is then referenced immediately to the aircraft heading, which reduces the necessity for mental calculation.